1. Field of the Invention
The invention relates to detecting the wax crystallization temperature in a waxy solvent. More particularly, the invention relates to on-line determination of the temperature at which wax crystals form in a dewaxing solvent stream, which comprises passing a solvent sample into a batch chiller into which a laser beam is emitted and cooling the solvent until wax crystal formation reflects the laser beam.
2. Background of the Invention
Higher molecular weight hydrocarbon fractions having an initial boiling point in the 550-650xc2x0 F. range typically contain wax, irrespective of whether the fraction is derived from natural or synthetic sources. Most wax containing hydrocarbon fractions are derived from naturally occurring sources, such as petroleum, bitumen and the like, but in the future more and more will be derived from synthetic crudes and hydrocarbon fractions produced by processes such as gas conversion, wherein natural gas or a gas comprising primarily methane is converted to a synthesis gas which, in turn, is used to synthesize hydrocarbons. Hydrocarbon fractions boiling in the range of from about 550-650xc2x0 F. to about 1050xc2x0 F. are used for lubricating oils for motor vehicles, turbines, machining and the like. In order for a lubricating oil fraction to be useful as a lubricating oil base stock, the wax must first be removed. This is accomplished by either solvent dewaxing or catalytic dewaxing, as is known. Most dewaxing facilities still employ solvent dewaxing, in which a chilled dewaxing solvent is slowly mixed with the lubricating oil fraction and the mixture slowly cooled, under conditions of agitation, down to the desired cloud or pour point temperature. As the mixture is cooled, wax crystals precipitate out, to form a slurry of wax crystals in the cold mixture of solvent and oil. Adding dewaxing solvent to the waxy oil also lowers the viscosity of the mixture. In many cases, a mixture of a wax solvent, such as toluene, and a wax antisolvent, typically comprising ketones such as methyl ethyl ketone and methyl isobutyl ketone, are used to reduce the solubility of the wax in the oil while avoiding oil immiscibility at the wax separation temperature. The wax is typically separated from the mixture of oil and solvent by filtration using rotary vacuum filters. The oily filtrate and wax precipitate are passed to separate fractionaters, to separate and recover the dewaxing solvent from the dewaxed oil and the wax. The hot dewaxing solvent recovered from the fractionaters is passed to indirect heat exchangers referred to as chillers, to lower its temperature sufficient for dewaxing. This temperature is lower than the dewaxing temperature. Wax entrained or carried over with the solvent in the solvent recovery fractionaters often causes fouling in the downstream dewaxing solvent chillers. The fouling comprises wax precipitation and the formation of a layer of wax on the interior heat exchange surfaces of the chillers, which acts as thermal insulation. As a consequence, the temperature of the dewaxing solvent exiting the chillers becomes too high for the downstream dewaxing operation. The chillers must then be taken off-line and cleaned, and this reduces plant capacity. A common way of checking for wax in the chilled solvent is for an operator to take a sample of hot solvent upstream of the chillers, slowly cool it, and visually determine the temperature at which wax crystals form. This is far from perfect. There is no control over the conditions. Taking a sample of hot solvent can be a fire hazard and, further, solvent evaporation while talking the sample can produce an artificially high wax crystallization temperature. In order to make the determination, the sample must be brought to a laboratory or other facility to make the determination. The hot sample is then slowly cooled and the temperature monitored, while watching for wax crystal formation. The temperature at which wax crystals begin to form is taken as the wax crystallization temperature. This method takes too much time to be useful for on-line or real time monitoring. Therefore, it would be advantageous to have a controlled and relatively quick, on-line detection technique capable of detecting the presence of wax in solvent down to a very low ppm. This would enable an operator to take corrective measures before the chillers became fouled and thereby maintain plant capacity.
The invention relates to a method for determining the temperature at which wax starts to crystallize out of a wax-containing dewaxing solvent upstream of a solvent chiller, without exposing it to the ambient. The method comprises taking a slipstream of the solvent upstream of the chiller and passing it into a sample chamber, into which a laser beam is emitted. The solvent sample containing the dissolved wax is then cooled, preferably under conditions that provide relative motion between it and the beam. The solvent temperature is recorded as it is cooled. As wax crystals start to form in the solvent solution, they scatter and reflect the laser beam striking them, as they pass through it. The reflections are detected and indicate wax crystal formation. The temperature at which the wax crystals begin to form is noted and recorded as the wax crystallization temperature of the sample. The sample chamber, means for passing solvent from the solvent line into the sample chamber, the laser, cooling means and temperature detecting means may be part of a solvent loop attached and adjacent to the dewaxing solvent line. The entire procedure may be accomplished automatically from a remote station. While useful as an on-line method for determining, upstream of a solvent chiller, the wax crystallization temperature of a hot, waxy solvent stream recovered from a wax-solvent and/or oil-solvent fractionater downstream of a rotary vacuum wax filter, the invention is not intended to be so limited. In its broadest sense, the invention comprises a method for determining the temperature at which wax crystals form in a solution of wax dissolved in a solvent (hereinafter xe2x80x9cwaxy solventxe2x80x9d), wherein the method comprises passing a waxy solvent sample into a batch chiller into which a laser beam is emitted and cooling the solvent until wax crystal formation reflects the laser beam. In another embodiment it comprises determining if wax crystals will form in a solvent at or above a particular temperature, by cooling a solvent free of wax crystals down to the temperature in the presence of a laser beam and noting if wax crystal formation has occurred at or above the temperature, as determined by whether or not the laser beam has been reflected at or above the temperature. If a solvent has wax crystals in it and it is desired to determine the wax crystallization temperature of the solvent, it must first be heated to a temperature high enough to insure complete solution of the wax. In yet another embodiment, the process comprises contacting a waxy oil with cold dewaxing solvent to form a wax precipitate and a dewaxed oil, heating the dewaxed oil and wax and passing them to separate fractionaters to separate the solvent from the wax and oil, passing the hot solvent recovered from the fractionaters to solvent chillers to cool the recovered solvent down to the dewaxing temperature and recycling the cooled solvent back to the solvent dewaxing operation, wherein a sample of the hot solvent being passed to the chillers is cooled to a predetermined temperature in the presence of a laser beam and determining whether or not wax crystals form at or above the predetermined temperature. The predetermined temperature will be somewhat lower than the temperature to which it is desired to cool the solvent in the chillers. If wax crystals do form, then corrective measures are taken upstream to change the conditions in one or more fractionaters, to insure that wax will not form in the solvent in the downstream chillers. When used with a focused, visible light laser beam, this method has determined the formation temperature of wax crystals in a waxy solvent, in which the wax concentration was as low as 12.5 wppm.